Do you mean "Which current denomination is most like the early Christian Church?" In that case, to a large degree, it's a theological question about which points of similarity are the most important - e.g. is it important to be subordinate in some sense to the Bishop of Rome, or to use leavened bread for the Eucharist, or a number of other things. The Roman Catholic and Eastern Orthodox churches would each claim to be the clear successor of the early Church; a number of Protestant groups would also claim to be substantially similar.

Or, are you asking "What was the first Christian group to claim to not just be Christian but to be a specific denomination within Christianity?" In that case, the answer would probably either be the Ebonites (a "Judaising" group which taught that people needed to still follow the Old Testament law), or the Gnostics.